Britons eat more junk food than ever before. But a forthcoming report by the top medical advisor says rising obesity is also due to a lack of exercise. Francis Elliott reports.

It is Britain's guilty little secret: every day and in every way we are getting fatter and fatter.

It is Britain's guilty little secret: every day and in every way we are getting fatter and fatter.

If Christmas lunch contained a health warning, it would read something like this: "It will take an 18-mile hike to burn off all this fat. Enjoy!" That's how far nutritionists calculate a man of average build needs to walk to use up the thousands of calories contained in a decent yuletide blowout.

So while that gentle stroll round the park may have made us feel a little less bloated, it barely cleared the mince pies. Britain's expanding waistline has become a national obsession, but so far most attention has been focused on our changing diet. We are eating more fast-food meals (two billion a year) and drinking more alcohol and sugary drinks. Sales of sweets and snacks in Britain outstrip those in other European countries by a wide margin.

Public health experts say this is only half the story: the fact is that we have never been so inactive. Six out of 10 male adults don't manage even half an hour's moderate activity five days a week while women are even less likely to do enough exercise. The consequences are dramatic: a fifth of men and a quarter of women are obese. The number of adults weighing more than they should now stands at 24 million.

Figures released to the House of Commons library earlier this month confirmed the "fat Britain" trend. The problem is worst among men in the North-east, says the official Health Survey, followed by the West Midlands and Yorkshire and Humberside. For women it is the East Midlands that has the highest average body mass index (BMI). Londoners may have the lowest BMIs for both genders but the average resident of the capital is still overweight.

It is the statistics among children that really scare health experts. Not only has the number of obese six- to 15-year-olds risen in the past five years, clinicians report they are routinely seeing weight-related diabetes in children, a condition associated with adults. A Department of Health report, Physical Activity and Health, to be published in spring, confirms that Britain's youngsters are a "couch potato" generation. Parental concerns about letting children play outside is one reason that Sir Liam Donaldson, the department's Chief Medical Officer, identifies for declining levels of physical activity. A decline in school physical education and more time watching television have also been blamed. Children are four times more likely to be overweight if they watch more than five hours a day.

The human cost is huge, with increased risks of heart disease, diabetes, cancer and depression reducing average life expectancy by nine years.

The cost of the crisis is helping to concentrate minds in Whitehall. Dealing with a diabetes epidemic could become the largest item on the NHS bill, some experts say: health economists already estimate the obesity crisis could cost the health service £3.6bn a year by the end of the decade.

But ministers know they must tread carefully. The links between social class and weight make a tricky health message even more difficult to get across. Some experts also suspect the Government is switching the focus to physical activity because it is scared to take on the food industry. Tessa Jowell, the Secretary of State for Culture, rejected an advertising ban on junk food, and ministers are said to be cool on a proposal to require health warnings on snacks.

Paul Burstow, the Liberal Democrat health spokesman, said: "There is no escaping personal responsibility for weight control but the Government must help people make informed choices. That means more informative labelling and pressure on those in the food industry who fail to be honest about the content of their product."